Use the sheet and 20mm test cube to calibrate your stepper motors.
Stepper motors should be calibrated every time you service your printer.
For those of you who have never calibrated your stepper motors before you will need to download repetier host in order to do this, once you open the software and connect your printer to the PC and the software, click on the "config tab" then click on the "firmware EEPROM configuration tab" in order to change the values of your stepper motors.
Use the provided sheet and follow the formula at the top of the sheet.
Target size X current steps per millimetre ÷ measure size =